Subaru Infrared Spectroscopy of the Pluto

Several previous observations of Pluto in the J, H, and K bands have derived its surface composition of mixture ice of N2, CH4, and CO. Hydrocarbons generally have a fundamental C-H stretching mode in the L band (2.8 – 4.0 um), the absorption of which is much stronger than the overtones in the J, H, and K bands. However, it is difficult to make precise spectroscopic measurements in the L band owing to the high background and variable telluric extinctions. So the spectral resolution of previous data of Pluto in the L band was insufficient to examine the absorption features of hydrocarbons of Pluto’s surface.

We have conducted an infrared spectroscopy of the Pluto-Charon system in the L band with the adaptive optics system on the Subaru telescope on 2002 May 28. Thanks to the adaptive optics system, we were able to derive the high resolution data of Pluto. The spectra is dominated by the strong and broad absorptions of methane, but include some additional features between 3.05 and 3.9 um possibly due to hydrocarbon molecules other than methane. Comparing the observed spectra and simple model calculations by Hapke’s bidirectional model, we considered the effects of some hydrocarbon molecules to the shape of the spectrum of Pluto. We discussed the evolution of components on Pluto through some processes (non-equilibrium condensation, photochemical reaction, cosmic-ray irradiation, hydrodynamic escape, and external reservoir’s addition), and predicted the second major components (HCN, C2H2, C2H6), which was consistent with our observation.

Observational spectrum compared with two model spectra.

Solid curve: the observation smoothed by running average of 31 pixels.
Dotted curve: synthetic spectrum of N2-CH4-CO ices, those CH4 ices are diluted in N2 ices.
Dashed curve: synthetic specctrum of N2-CH4-CO ices and C2H2:CH4 = 1:30, C2H6:CH4 = 1:10 with diluted CH4 ices.

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